The present invention relates generally to chemical vapor deposition (CVD) processing, and more particularly to a method and apparatus for CVD chamber cleaning.
CVD is widely used in the semiconductor industry to deposit films of various kinds, such as intrinsic and doped amorphous silicon (a-Si), silicon oxide (SixOy), silicon nitride (SirNs), silicon oxynitride, and the like on a substrate. Modem semiconductor CVD processing is generally done in a vacuum chamber by using precursor gases which dissociate and react to form the desired film. In order to deposit films at low temperatures and relatively high deposition rates, a plasma can be formed from the precursor gases in the chamber during the deposition. Such processes are known as plasma-enhanced CVD processes or PECVD.
State of the art CVD semiconductor processing chambers are made of aluminum and include a support for the substrate and a port for entry of the required precursor gases. When a plasma is used, the gas inlet and/or the substrate support is connected to a source of power, such as a radio frequency (RF) power source. A vacuum pump is also connected to the chamber to control the pressure in the chamber and to remove the various gases and particulates generated during the deposition.
In all semiconductor processing, particulates in the chamber must be kept to a minimum. Particulates are formed because, during the deposition process, the film is deposited not only on the substrate, but also on walls and various fixtures, e.g., shields, the substrate support and the like in the chamber. During subsequent depositions, the film on the walls, etc., can crack or peel, causing contaminant particles to fall on the substrate. This causes problems and damage to particular devices on the substrate. Damaged devices have to be discarded.
When large glass substrates, e.g., of sizes up to 360 mmxc3x97450 mm or even larger, are processed to form thin film transistors for use as computer screens and the like, more than a million transistors may be formed on a single substrate. The presence of contaminates in the processing chamber is even more serious in this case, since the computer screen and the like will be inoperative if damaged by particulates. In this case, an entire large glass substrate may have to be discarded.
Thus, the CVD chamber must be periodically cleaned to remove accumulated films from prior depositions. Cleaning is generally done by passing an etch gas, particularly a fluorine-containing gas, such as nitrogen trifluoride (NF3), into the chamber. A standard method of performing this cleaning procedure is to pass a constant flow of NF3 into the chamber. A plasma is initiated from the fluorine-containing gas which reacts with coatings from prior depositions on the chamber walls and fixtures, e.g., coatings of a-Si, SixOy, SirNs, SiON and the like, as well as any other materials in the chamber. In particular, the NF3 creates free fluorine radicals xe2x80x9cF*xe2x80x9d which react with Si-containing residues.
The reaction forms gaseous fluorine-containing volatile products that can be pumped away through the chamber exhaust system. This procedure is generally followed by a nitrogen purge.
As the volatiles are pumped away, the F* contribution to the overall pressure stays low until there are no residues left with which the F* can react. Then the F* contribution to the overall pressure, and thus the overall pressure itself, rises. One can use this rise to detect the endpoint of the cleaning procedure by monitoring the overall pressure with a manometer and waiting for a user-defined endpoint pressure to be reached.
This technique is vulnerable to, e.g., variations in the manometer pressure reading over time, i.e., xe2x80x9cmanometer pressure driftxe2x80x9d. Such variations are inherent in any such instrument and may be typically provided for with appropriate calibration techniques. If the variation is such that the manometer drifts low, the process may not seem to attain the endpoint. In this situation, the cleaning recipe is carried out indefinitely, or until a preset time is reached, termed a xe2x80x9crecipe timeoutxe2x80x9d. Even though the chamber is cleaned, throughput is reduced and cost increased, the latter especially because NF3 is significantly more expensive than that of any other process gas. Flowing a large and constant amount of NF3 is costly and inefficient.
On the other hand, if the manometer drifts high, the preset pressure may be attained prior to endpoint, resulting in an incomplete clean. Residues may be left in the chamber that are a potential source of contaminant particles.
One way to conserve NF3 is to install a governor on the valve inlet of the NF3. The governor allows NF3 flow in such a manner as to keep the pressure of NF3 constant within the chamber. In this way, the flow rate is no longer constant, decreasing near the endpoint and saving costly NF3. A drawback of this solution is that the endpoint of the cleaning procedure cannot be detected using the rise of the NF3 level, since its pressure is by design constant.
In one aspect, the invention is directed to a method for cleaning a processing chamber including optoelectronic detection of the completion or endpoint of the cleaning procedure once a ratio of emission lines reaches a threshold value. The method comprises the following steps. A plasma of a cleaning gas is provided in the chamber. The intensity of emission lines of at least one cleaning gas and of the background emission in the chamber are monitored. A ratio of the intensity of the cleaning gas emission line to the intensity of the background emission is determined and monitored over a period of time. The determined ratio is compared to a preset threshold calibration value. The flow of cleaning gas is controlled based on the comparing step.
In another aspect, the plasma of the cleaning gas at least partially dissociates a portion of the molecules of the cleaning gas. The emission line intensity of a constituent of the partially dissociated cleaning gas may be monitored and used in a ratio as above.
Implementations of the invention may include one or more of the following. The chamber may be a CVD chamber. The cleaning gas may be NF3 with a partial pressure in a range of about 0.1 and 2.0 Torr, and more particularly about 1.0 Torr. The constituent of the partially dissociated cleaning gas may include fluorine. The cleaning gas emission line monitored may be at about 704 nanometers. The intensity of emission of the background gas may correspond to the intensity of emission of a plurality of background gases as measured through a neutral density filter. The controlling step may include steps of starting a preset delay period based on the comparing step, and controlling the flow of gas after the end of the preset delay period. The preset delay period may be implemented in hardware or software or both.
In another aspect, the invention is directed to a cleaning system for a processing chamber, comprising a cleaning gas supply with a valved inlet providing an entrance to the interior of the chamber for passing a cleaning gas to the interior of the chamber. A detector having an optical input is used to sense electromagnetic radiation in the interior of the chamber. The detector has a first channel for detecting a relative intensity of an emission line corresponding to a constituent of the cleaning gas and a second channel for detecting a relative intensity of an emission line corresponding to the background gases. A means is employed to determine a normalized signal using a signal from the first channel and a signal from the second channel. The value of the normalized signal is substantially invariant with respect to simultaneous corresponding changes in the intensity of the signal measured by the first channel and the intensity of the signal measured by the second channel.
Implementations of the invention may include the following. The system may further comprise a neutral density filter for filtering an input to the second channel such that the second channel measures the relative intensity of emission lines of an aggregation of the background gases. The means may be software or circuitry or both. The system may further comprise a timer which institutes a preset delay period. The timer may be instituted in hardware or software or both. The detector may be disposed on the exterior of the chamber to monitor radiation in the interior of the chamber through a transparent viewport, or may be directly placed in the interior of the chamber.
In another aspect, the invention is directed to a method for cleaning a processing chamber including optoelectronic detection of the completion or endpoint of the cleaning procedure once a ratio of emission lines reaches a threshold value. The method includes the following steps. A plasma of a cleaning gas is provided in the chamber. The intensity of emission lines of the cleaning gas and of at least one background gas in the chamber are monitored. A normalized signal is determined using the intensity of the emission line of the cleaning gas and the intensity of emission of the background gas, such that the value of the normalized signal is substantially invariant with respect to simultaneous corresponding changes in the background gas and cleaning gas signal intensities. The normalized signal is monitored over a period of time. The normalized signal is compared to a preset threshold value. The flow of gas is controlled based on the comparing step.
An advantage of the invention is that a significantly reduced amount of cleaning gas is necessary to clean a chamber because only the amount of NF3 actually necessary to clean the chamber is used. Less waste of costly NF3 occurs because real-time monitoring of the cleaning is employed. It is another advantage that the endpoint of a chamber cleaning can be determined. For at least these reasons, the cleaning process is less costly than prior methods. Finally, problems with manometer drift and recipe timeout are substantially eliminated.